Demagnetizer for magnetic recorder-reproducer



Oct. 27, 1953 w. A."BRASTAD DEMAGNETIZER FOR MAGNETIC RECORDER-REPRODUCER 2 Sheets-Sheet 1 Filed Aug. 30, 1946 \l\ INVENTOR I WILLIAM A. BRASTAD ATTY Oct. 27, 1953 w. A. BRASTAD DEMAGNETIZER FOR MAGNETIC RECORDER-REPRODUCER 2 Sheets-Sheet 2 Filed Aug. 30, 1946 INVENTOR WILLIAM A. BRASTAD mm W W. W m E h p E s mt m HUME ATTY Patented Oct. 27, 1953 DEMAGNETIZER FOR MAGNETIC RECORDER-REPRODUCER William A. Brastad, Minneapolis, Minn., assignor to General Mills, Inc., a corporation of Delaware Application August 30, 1946, Serial No. 694,123

27 Claims.

This invention relates to a method of and means for demagnetizing the field core of the recording head as used for recording sound on wire; In nearly all such recordings, there is some residual orremnant magnetism retained in the pole pieces and field cores depending, of course, for the amount on the particular point in the magnetization curve at which the switch opens in the recording circuit. This residual magnetism if left in the recording head, will often seriously interfere with the operation of that same head when used as a reproducer, by causing a hissing or crackling background noise. In some cases the residual magnetism may even act to record noise on the wire which is being played back through the head, due to mechanical movement of the wire in these magnetized pole pieces. This noise would be left on the wire even though no attempt were made to use this magnetized head as a reproducer.

An object of this invention, therefore, is to provide a method of and apparatus for removing this residual magnetism in the recording head and particularly to do this automatically on switching off the audio recording'current as each recording ends.

One method of demagnetizing objects is to apply an alternating magnetic field and gradually reduce the magnitude of these alterations to zero. Such an alternating field is readily available in the heads of magnetic recorders which employ a supersonic biasing flux to obtain approximately linear response to audio frequency. signals.

Another object of this invention is to provide a method of and apparatus for attenuating the supersonic biasing flux in'the core of the recording head so as to leave the head in a demagnetized condition for use in reproducing.

Demagnetizing of the core of the recording head can also be accomplished by attenuation of certain audio frequency fluxes or combinations of alternating bias fluxes and audio frequency the supersonic oscillator has been cut off in accordance with this invention;

Fig. 2 is a wiring diagram showing the parts and their connections for utilizing this invention; and,

Figs. 3 and 4 are modifications of Fig. 2.

Like numerals indicate like parts in the differ ent figures.

The graph of Fig. 1 shows at A a sine curve plotted between voltage across the recording coil as ordinates with time as the abscissa. The part of the curve labeled B shows the attenuation of the alternating supersonic voltage when the usual power source is cut off as will later be described. It is the diminution of the amplitude of this sine wave, as shown at B, which brings about the desired demagnetization of the recording head. This declining supersonic voltage across the recording coil corresponds to what is commonly termed a damped oscillation.

The apparatus as shown in Fig. 2 comprises principally a'microphone lil, an audio amplifier l2, a direct current power source Hi, a vacuum tube oscillator I6 for supplying a supersonic biasing voltage to a recording-reproducing head It for magnetically'recording sound on wire and later reproducing the same, and a loud speaker 26, the connections of which are controlled through a multiple switch having a knob 39 on a shaft 32, which carries a series of switch arms 34, 36, 38, Ail and 42 insulated from each other and having a sliding electrical connection with a series of insulated switch rings, 44, 46, 48, 5t and 52 respectively.

The power source hi connects on one side to ground and on the other through leads 59, til to a metal button 62, which make electrical contact with the arm 34,'as shown in full lines in Fig. 2, while the lead 69 connects through a lead 6! to the audio amplifier i2 and thence through a lead 63 to the ground. Current from the ring is is connected through a lead E l withone side of the condenser 66, the opposite side of which is grounded. This lead is also connected to a tap 68 on a primary coil 10 of a transformer forming part of the oscillator it. The circuit of this type of vacuum tube oscillator is well understood and therefore will not be described in detail except to state that terminals 61-69 are direct current powerterminals of the oscillator. v

The secondary coil 72 connects at one end through a lead M with the metal switch ring 5i]; thence through the switch arm it to a bar Figure ,1 is a graph showing the supersonic biasing voltage, as applied to the recording head, plotted as a sine curve and showing the attenur ation of the voltage after the power suppliedto 16; and thence through a lead it, which is the output lead of the audio amplifier i2 the other lead of this amplifier output being grounded.

The input of the audio amplifier consists of a ground lead 80 and a lead 82, which connects with a metal ring 48 on the switch and thence through the arm 38 to a metal button 84, which connects through a lead 36 with the microphone H the other side of the microphone being grounded.

Returning to the secondary coil of 'the transformer of the oscillator 16, the other side of the coil 12 connects through a lead 90 to the ring 46 on the switch and thence through the arm 38 to a bar 92, which connects through a lead 94 to one end of a coil 96 on the recorder-reproducing head IS, the opposite end ofthe coil being grounded. This head is provided with suitable pole pieces through which passes a fine steel wire W, the wire passing in one direction for recording or reproducing and in the opposite direction for rewinding, in a. manner well known in this art. V V

The switch shows three positions; the one at the left being shown in full lines is the recording position; the one to the extreme right at 90 from the present recording position is the position for reproducing, but this is not described in this specification for the reason that it forms no part of the present invention. The intermediate position, in which the switch arms 34 to 42 are shown dotted in the upright position, is the one with which this specification is particuiariy interested, and that position and its effect on the recording head will now be described in detail.

It will be noted that of the five switch arms, only arms 36 and '48 contact bars 92 and "is respectively, rather than single contact buttons. This enables these switch turns to remain in contact with these bars as the switch is rotated from the recording to the intermediate position, thereby enabling the two "ends of the secondary coil 12 to remain connected to ground through the leads 1'4, 4 0, '18, and the audio amplifier on the one hand and through the leads 9D, the switch arm 315, the bar 92, an'd the lead 95 to the coil 95 of the recorder head 13, and thence to ground.

Thus as the switch is turned to this intermediate position, the power source 14 is cut off from the oscillator lead Turning the switch to the intermediate position, while it thus cuts off the power source, leaves the condenser 0K5 still conn cted into the oscillator lead es, With the result as shown in Fig. 1 that the electric energy stored up in the condenser '06, rapidly bleeds on through the tap $8 to the primary winding 59 or the transformer, and the oscillator in turn continues to generate a voltage, which, however, is rapidly decreasing in amplitude as shown in Fig. l, in the secondary winding 12-, and this is applied through the lead 90, the ring the arm as, the bar 92, and the lead at, to the recording coil 96 of the recording head. The circult is completed through the grounded leads to the audio amplifier output, the lead is, l8, arm '40, ring and lead [4.

Referring to Fig. l, the full supersonic voltage is shown diagrammatically as a sine wave in the left side of this figure and up to the point X where the switch knob 30 is moved to the intermediate dotted position. The attenuating voltage is shown by the curve to the right of this point X, and in about 0 of a second or less has fallen substantially to zero.

During this time, the magnetization of the core of the recording head has been reduced substantially to zero so that practically no residual mag= netization remains to cause trouble when the sound is again reproduced, as might be thecase if any substantial amount of residual magnetiza tion were left in the core. Oscillation frequencies in use for this purpose are of the order of 20,000 to 30,000 vibrations per second, so that while the tiine required to discharge the condenser 65 is quite short, it is still sufficient to permit quite a number of oscillations to occur before the charge on the condenser is substantially spent.

The oscillatory circuit constitutes a load across the condenser 6-8 and the time required for the condenser to discharge to a negligible value after this circuit is broken, is governed by the constants of the oscillatory circuit and the capacity of the condenser. The capacity of the condenser and the other "characteristics of the circuit are adjusted so that the time required to discharge the condenser is very short, say of a second. Any form of snap switch will suffice which insures such a time interval as the switch passes through the intermediate position.

Fig. 1 is shown with no audio component since the attenuating supersonic must preferably operate about the zero voltage axis. In practice this is readily accomplished by reducing the audio gain to Zero before switching to the iii-- termediate position, or the switch of Fig. 2 may be so designed that the contact with button '84 is interrupted shortly before contact is broken at button 62.

A modification of the apparatus of Fig. 2 is shown in Fig. 3 in which an extra lower section of the switch is added With a switch ring us and a switch arm on a shaft 132-, the arm engaging a contact button 15?. A lead [6| connects the power lead 59 to the switch ring M5 While the contact button '16? connects through a lead mm to the audio amplifier and thence to ground through the ground lead 83, condenser l 'ii connects with lead Ii-Hd through a lead N2, the other side being grounded; The electric energy stored in this condenser while the switch arm r55 makes contact with the button is? serves to supply an attenuating voltage to the audio amplifier when thearm I65 breaks contact with the button 16?. With the shaft 32 in the intermediate position, this attenuating voltage applied to the amplifier produces attenuation of any musical note or other sound received by the microphone it. The attenuation of this note has much the same effect on demagnetiz'ing the re-' cording head as does the attenuation of the volt age of the oscillator it of Fig, 2. While in Fig. 3 the two are operating simultaneously, it will be understood that either alone will serve the purpose. I

In Fig. 4 is shown another modification of the apparatus of Fig. 2 in which a shaft 232 of the switch carries an entra switch section having a switch ring 2'45 and a switch arm 265 adapted-to contact a button let.

A bar 85' replaces the button 8% of Fig. 2 so that the microphone as is connected to the amplifier for first and second positions of the switch. one side of the output of the power supply is connected to ground while the loads as, so-,anu 5: connect to the audio amplifier, and the lead fill connects to a bar 2E2. The switch ring 2 15 sects through a lead sea to one terminal or the power supply, while a lead 210 connects at one end to one side of the power line and at the other to a button 14! on the switch section, which in turn connects through a lead IE9 with the reproducing or third position button of the same section, thei-nte'rinediate button I53 being blank. This latter switch section thus serves as a switch controlling the power furnished from the power 1ine to the power-supply. In the second or intermediate position, the line will not be furnishing power to the power supply but it will be furnishing power at both end positions of the switch.

It will also be apparent that where the power supply contains a number of condensers and chokes to make up the usual filter circuit 266, they store electric energy and thus take the place of the condenser 66 of Fig. 2 so that where this type of filter circuit is used the condenser 66 may be omitted. Power stored up in this filter circuit will attenuate in much the same way as that described for the condenser 66.

The heaters of the oscillator and amplifier tubes are preferably maintained in connection with their energizing circuit while the demagnetizing of the core takes place in the intermediate switch position so that the amplifier will be ready for use immediately following movement of the switch to the reproducing position. As the condenser 65 and any other condensers in the power supply discharge their stored energy into the oscillator and audio amplifier, all alternating fluxes as in the recording head core will be gradually attenuated, thus bringing the care to a low degree of residual or remanent magnetization. If the pause in the intermediate position is sufficiently brief, the heaters of tubes employed will not cool enough to delay the reproducing operation.

Thus it will be seen that I have provided a very simple and efiicient means for demagnetizing the core of a recorder-head, and means which can be applied to a recorder of this type at very low cost.

While I have shown and described but a few forms of my invention, it is to be understood that it is capable of many modifications. Changes therefore may be made which do not depart from the spirit and scope of the invention as defined in the appended claims.

I claim as my inventions 1. A device for recording sound on magnetizable bodies comprising a core of magnetic material, means including anamplifier with a power source for applying to said core an audio frequency magnetic flux to be recorded, means including an oscillator also using said power source for applying to said core a high frequency biasing fiux, and means including a condenser con- 3. A device for recording sound on magnetizable bodies comprising a core of magnetic material, means for applying an audio frequency magnetic flux corresponding to sounds to be re.- corded to said core, an oscillator for applying to said core a supersonic biasing magnetic flux, a direct current power source connected to the oscillator, a switch located in the circuit between the direct current power source and the oscillator to terminate the application of direct current power to the oscillator, and a condenser connected across the direct current power supply terminals of the oscillator and located in the circuit between the switch and oscillator, the condenser maintaining the supersonic biasing fiux in the core and damping it substantially, to zero after the switch is operated to terminate the application of power.

4. A device for recording sound on magnetizable bodies comprising a recording'head having a core of magnetic material, an audio amplifier for supplying to the head an audio frequency voltage corresponding tosounds to be recorded. means for supplying a high frequency biasing voltage connected to the head,'a direct current power source connected to saidbiasing means, a switch located in the circuit between the power source and biasing means to terminate the application of power to said means, the biasing means remaining in connection to the head When' the switch is operated to terminate the application of power to said biasing means, and a power storage means connected across the power source terminals of the biasing means and momentarily continuing and damping the power supply to the biasing means, maintaining the biasing voltage and damping it substantially to zero after the switch is operated to terminate the application nected across the direct current power terminals 7 necting the power source to a supply line, a switch located in the circuit between said connecting means and said oscillator to terminate the application of powerto the oscillator, and a condenser connected across the power supply terminalsof the oscillator and located in the circuit between the switch and oscillator, said condenser maintaining the supersonic biasing fiux and damping .it substantially to zero after the switch'is opee sw r mee we n ie n'pip eof power. v

5. A device for recording sound on magnetizable bodies comprising'a recording head having a core of magnetic material, means for supplying an audio frequency voltage corresponding to sounds to be recorded to said head, an oscillator connected to the head for supplying to said head a high frequency biasing voltage, a direct current power source connected to they oscillator, a first switch located in thercircuit between the direct current power source and the oscillator and a second switch in the circuit between the oscilla-'- tor and head, the switches being connected for simultaneous movement between a first position in which the circuit between the direct current power source and the oscillator is closed and the circuit between the oscillator and head is closed and a second position in which the circuit between the direct current power source and the. oscillator is open, and the circuitbetween the oscillator and head is closed, and a condenser connected across the power supply terminalsrof the oscillator and located in'the circuit between the first switch and oscillator, the condenser maintaining thehigh frequency biasing voltage of the oscillatorin the head and damping it substantially to zero after the switch is moved to the second position toterminatethe application of power. 7

6. A device for recording sound on magnetizable bodies comprising a recording head having a core of magnetic -material,- an audio amplifier forsupplying an audio frequency voltage corresponding to sounds to be recorded to the head,

means for supplying an audio frequency voltage "corresponding to sounds to be, recorded to the amplifier, a direct current power source connect.

anamored to the amplifier, a Switch located in thcircilit bet can the power source and the amplifier to terminate the application or power to the ampliher, and a condenser connected across the power supply terminals of the amplifier and located in circuit between the switch and amplifier, the condenser maintaining the audio frequency voltage of the amplifier in the head and damping it substantially to zero after the switch is operated to terminate the "application Of power.

' 7. A device for recording sound on magnetizable bodies 'comprisingfa recording head having a core of magnetic material, an audio amplifier connected to the head for Supplying an audio freduncy voltage corresponding to sounds to be recorded to the head, means for supplying an audio "frequency voltage corresponding to sounds to be recorded to the amplifier, means for supplying a high frequency biasing voltage to the head, a direct current power source connected to the amplifier, a switch located in the circuit between the power source and the amplifier 'to terminate the appl'ica'tio'n or power to the amplifier, and electrical storage means connected across the direct current power terminals of the amplifier and located in 'th'ecirc'uit between the switch and amplifier, said storage means momentarily continuing and damping the voltage to the amplifier maintaining the "audio frequency voltage in the head and damping it substantially to zero after the switch is operated 'to terminate the application of power. I

8. A device for recording sound on magnetizable bodies comprising a recording head having a car er magnetic material, an audio amplifier for supplying 't'o "the head an audio frequency voltage corresponding to sounds to be recorded connectedtothe head, means for supplying an audio irequency voltage corresponding to sounds to be recorded to the amplifier, 'a' direct current power source connected to the amplifier, "a first switch located in the circuit between the power source and amplifier and a second switch located in the circuit between the amplifier and head, the switches being connected for simultaneous'move- *inentbetween a first position in which the cir- "cuits between "the amplifier and head and between the ainplifierand power source are closed and 'a second fpositi'c-n in which the circuit betweenthe amplifier and powersource is ope'nand the circuit between the amplifier and head is "closed, and a condenser "connectedfacross the power source terminals "of the amplifier and loc'ated between the amplifier and first switch, the condenser maintaining the audio frequency voltage in the head and damping it substantially to 'zero' after the'switch is operated to terminate the application of power.

9. A device for recording sound on magnetizable bodies comprising a recording head having a cor'eoi magnetic material, an audio amplifier connected to'the heacl for supplying an audio frequency "voltage to the head corresponding "to :sounds to be recorded, 'means for supplying an audio frequency voltage corre's'ponding to sounds.

to berecorded to the amplifier, an oscillator-connected to the head for supplying a high frequency biasing voltage to the head, direct current power source "connected to 'the oscillator and to the amplifier, a switch located in the circuits between the power source andamplifier and power source andoscillator'toterminate the application of power, and condenser means connected across the powersupplyterminals of the amplifier'and across the -power-siipplyterminals of the oscilla tor and located in the circuit between the ampli= her and switch and between the oscillator and switch, the condenser maintaining said audio frequency and high frequency voltages in the head and damping them substantially to zero after the switch is operated to terminate the application of power, the amplifier and oscillator continuing to be connected to the head when the switch is operated.

10. A device for recording sound on magnetizable bodies comprising a recording head having a core of magnetic material, an audio amplifier connected to said head for supplying an audio frequency voltage corresponding to sounds to be recorded to the head, means for supplying an audio frequency voltage corresponding to sounds to be recorded to the amplifier, an oscillator connected to the head for supplying to the head a high frequency biasing voltage, a direct current power source connected to the oscillator, a first switch located in the circuits between the power source and the oscillator and power source and amplifier, a second switch located in the circuit between the oscillator and head, a third switch located in the circuit between the amplifier and head, the switches being connected for simultaneous movement between the first position in which all of said circuits are closed and a second position in which the circuits between the power source and oscillator and between the power source and amplifier are open and the circuits between the head and amplifier and between the head and oscillator are closed, and condenser means connected across the direct current power terminals of the amplifier and oscillator and located in the circuit between the oscillatotand first-switch and amplifier and rust switch, said condenser maintaining the audio frequency and high frequency voltages in the head and damping them substantially to zero after the switch is operated to second position to terminate application of power.

11. A device for recording sound-on magnetizable bodies comprising a recording head having a core of magnetic material, an'audio-amplifier for applying to the head-an audio frequency voltage corresponding to sounds to be recorded, an oscillator connected to the head for "supplying a high frequency biasing voltage, a direct current power sou'rce coimec'ted to the oscillator, mea'ns for conne'ctingthe power source to a supply line, a first switch-located in the circuit between the supply line and power source and-a second switch located in'thecircuit betweenthe power source and oscillator, and a third sWi-tch located in the circuit between the oscillatorand head, the switches being connected'for-simultaneous movement between a first position in which 'said circuits are closed and a second position in which the circuit between the-"Supply line and ower source -is o en and-the circuits between the power source and oscillator and between the oscillator and head are jclos'edfthe high frequency biasing voltagedamping substantially 'to zero in the head after the switch is operated from the first to second posi- 'tion.

12. A'device-for --r'eco'rding sound on magnetizable bodies comprising arecording head having a 'core of'magneticmaterial, means for supplying an audio frequency voltage corresponding to sounds to be r'ecordedto the head, an oscillator connected to the nead'fcr supplying a "high frequency biasing fvoltage-"a direct current power source connected to the oscillator, a multiple ang switch "having a series of "switch units naving at least two positions, one unit located in the circuit between the oscillator and head and having an electrical connection between the first and. second positions, both positions closing the circuit, the second unit located in the circuit between the direct current power source and the oscillator and closing said circuit in first position and breaking said circuit in second position, and a condenser connected across the power source terminals of the oscillator and located in the circuit between the oscillator and second switch unit, said condenser maintaining the high frequency biasing voltage of the oscillator in the head and damping it substantially to zero after the gang switch is operated from first to second position to terminate the application of power.

13. A device for recording sound on magneticable bodies comprising a recording head having a core of magnetic material, amplifier means con- 'nected to the head for supplyingan audio frequency voltage corresponding to sounds to be recorded, an oscillator means connected to the head for supplyin a high frequency biasing voltage, a direct current power source connected to the audio amplifier and to the oscillator, means for connecting the power source to a supply line, a first switch located in the circuit between the power line and one of said voltage supply means and having a first position in which. the circuit is closed and a second position in which the circuit is open, a second switch located in the circuit between the head and the said voltage supply means and having first and second positions in which the head and the said voltage supply means remain connected to each other, the second switch having a third position in which the head and voltage supply means are disconnected from each other, the first and second switches connected for simultaneous movement between their respective first and second positions, and a condenser connected across the power supply terminals of the said voltage supply means and located in the circuit between the first switch and the said voltage supply means, the condenser maintaining the voltage and damping it to zero in the head after the switch is operated from first to second position to terminate the application of power.

14. A device for recording sound on magnetizable bodies comprising a head having a core of magnetic material, an audio amplifier having signal input and output sides, means for supplying to the amplifier an audio frequency Voltage corresponding to sounds to be recorded, means for receiving and reproducing to sound an audio frequency voltage from the amplifier corresponding to recorded sounds, an oscillator connected to the head for supplying a high frequency biasing voltage, a direct current power source connected to the oscillator, a multiple gang switch having three positions and including a first unit located in the circuit between the power source and oscillator and closing said circuit in first position and opening said circuit in second and third positions, and a second unit located in the circuit between the oscillator and the head and closing said circuit in first and second positions and opening said circuit in third position, the remaining units connecting the audio voltage supply means to the input side of the amplifier and the output side of the amplifier to the head in first position, and connecting the head to the input side of the amplifier and the output side of the amplifier to the reproducing means in third position, and a condenser connected across the direct current power supply terminals of the oscillator and located between the oscillator and first switch unit and maintaining the high frequency biasing voltage and damping it substantially to zero after the switch is operated from first to second position.

15. A device for recording sound on magnetizable bodies comprising a head having a core of magnetic material, an audio amplifier having signal input and output sides, the output side connected to the head, means for supplying to the input side of the amplifier an audio frequency voltage corresponding to sounds to be recorded, means for receiving and reproducing to sound an audio frequency voltage from the amplifier corresponding to recorded sounds, a direct current power source connected to the amplifier, a multiple gang switch having three positions and including a first unit located in the circuit between the power source and amplifier and closing said circuit in first position and third position and opening said circuit in second position, a second unit located in the circuit between the amplifier output side and the head and closing said circuit in first and second positions and opening the circuit in third position, the remaining units connecting the audio voltage supply means to the input side of the amplifier in first position and connecting the head to the input side of the amplifier and the output side of the amplifier to the reproducing means in third position, and a condenser connected across the power source terminals of the amplifier and located between the amplifier and first switch unit, the condenser maintaining an audio frequency voltage in the head and damping it substantially to zero afterthe switch is operated from first to second position.

' ing to the amplifier an audio frequency voltage corresponding to sounds to be recorded, means for receiving and reproducing to sound an audio frer the oscillator, means connecting the power source to a supply line, a multiple gang switch having X three positions and including a first unit located in the circuit between the supply line and power source and closing said circuit in first and third positions and opening said circuit in second position, a second unit located in the circuit between the oscillator and head and closing the circuit in first and second positions and opening the circuit in third position, the remaining units connecting the audio voltage supply to the input side of the amplifier and connecting the output side of the amplifier to the head in first position and in third position connecting the head to the input side of the amplifier and connecting the output side to the reproducing means, and a condenser connected across the power source terminals of the oscillator and maintaining the high frequency biasing voltage in the head and damping it substantially to zero after the switch is operated from first to second position.

17. A device for recording sound on magnetizable bodies comprising a head having a core of magnetic material, an audio amplifier means for supplying an audio frequency voltage having a.

" signal input and output side, means for supplying to the amplifier an audio frequency voltage correarts?? spending to sounds to be recorded, means for receiving and reproducing to sound an audio frequency voltage from the amplifier corresponding to recorded sounds, an oscillator means connected to the head for supplying a high frequency biasing voltage, a direct current power source connected to the oscillator and to the amplifier, means connecting the power source to a supply line, a multiple gang switch having three positions including a first unit located in the circuit between the supply line and one of said voltage supply means and closing said circuit in first and third positions and opening said circuit in second position, a second unit located in the circuit between the head and the said voltage supply means and closing the circuit in first and second position and opening the circuit in third position, the remaining units connecting the other of said voltage supply means to the head in first position and disconnecting it in third position and connecting the head to the input side of the amplifier and connecting the output side of the amplifier to the reproducing means in third position, and a condenser connected across the direct current power supply terminals of the said voltage supply means and located in the circuit between said means and the first switch unit, the condenser maintaining the voltage and damping it substantially to zero in the head after the switch is operated from first to second position.

18. In a magnetic recording and reproducing system a recording and reproducing head, an, energ source coupled to said head for exciting said head with oscillations at a predetermined constant frequency during recording, and means for conditioning said head for reproducing comprising means for progressively reducing the intensity of said oscillations from said source, and means operative after said oscillations have been substantially reduced for decoupling said source from said head.

19. A magnetic transducer comprising a common electromagnetic recording and reproducing head, means for energizing said head with a varying current representing the signals to be recorded, means for concurrently energizing said head with alternating current 'at a frequency above the frequency band of said signals, and means for operating said head as a reproducer. comprising means coupled to said energizing means for progressively reducing the intensity ofsaid current, and means for disconnecting said energizing means from said head after said current has been substantially reduced.

20. In a magnetic recording and reproducing head arranged for recording with a biasing signal of supersonic frequency supplied by a source, said source being oper'ative'ly charged from a power supply to supply said signal, the method of preparing said head for reproducing comprising the sequential steps of removing said power supply from said source to initiate decay of said biasing signals, and disconnecting said head from said source, after said signals have decayedto substantially zero intensity.

21. A magnetic recorder comprising an electromagnetic recording head, an oscillator connected to provide an alternating current biasing signal to said head, energy storage means associated with said oscillator, a source of operating potential for said oscillator, and means for disconnecting said source from said: oscillator for, thereafter disconnecting said oscillator from said head, whereby operation of said last means 12 causes said oscillator to supply biasing signals of reducing intensity to said head during the interval'afte'r disconnection of said source from said oscillator and prior to disconnection of said oscillater from said head;

22. In a magnetic recording head arranged for simultaneous excitation by audio signals froma signal source and by supersonic biasing signals from a biasing signal generator, switch means for disconnecting said signal source from said head, and commonly operated second switch means for initiating substantially simultaneously decay in intensity of said supersonic signals.

' 23. In a magnetic recording head arranged for simultaneous excitation by audio signals from a signal source and by supersonic biasing signals from a biasing signal generator, switch means for disconnecting said signal source from said head, commonly operated second switch means for initiating substantiall simultaneously decay in intensity of said supersonic signals, and commonly operated third switch means operative after said last means for disconnecting said genera tor from said head.

24. In a magnetic recording head arranged for simultaneous excitation by audio signals from a signal source and by supersonic biasing signals from a biasing signal generator, means for disconnecting said signal source from said head, and means for initiating decay in intensity of said supersonic signals and for disconnecting said generator from said head after said decay has progressed a substantial amount.

25. In a magnetic recorder comprising an electromagnetic recording head excited by a supersonic biasing signal of a given intensity from a signal source, the method of deenergizing said head comprising the steps of progressively reducing the intensity of said supersonic signal, and decoupling said source from said head after said signal has been reduced a substantial amount.

26. In a magnetic recorder comprising an electromagnetic transd ucin g head arranged to be excited by a supersonic biasing signal from a signal generator, themethod of deenergizing said head comprising the step of gradually disabling said generator to provide said supersonic signal at decaying intensity to said head, and disconnecting said generator irorn said head after said signal has decayed an amount to preclude the existence of any substantial residual magnetism due to said supersonic signal in said head.

' 27. The method of preparing an electromag netic recording and reproducing head for the reproducing t n a 2 3%. f a re qr l s period d n w i a d and as en ex ite with supersonidbiasing signals from a signal source, which comprises the steps of decreasing gradually the intensity oi said biasing signals supplied to said head, and thereafter disconnectin said head from said source.

WILLIAM A. BaasT D.

References Cited in the file of this patent U I ED stare ATENTS 

